1. Field of the Invention
The present invention generally relates to an antilock control system for a motor vehicle, and more particularly, to an antilock control system wherein the wheel speeds of a motor vehicle are detected to judge whether the wheels are in the way of the lock-up process or the antilocking process, thereby to control the hydraulic brake pressure, so that the problem caused when a wheel of a smaller diameter is mounted in one of a plurality of wheels of the motor vehicle in emergency etc. is solved.
2. Description of Related Arts
According to a conventional arrangement of the type referred to above for the antilock control, a wheel speed sensor is mounted in association with each of a plurality of wheels (generally, front and rear wheels 4 in total) which generates a signal indicating the wheel speed and outputs the same to a control device. The wheel speed of each wheel is calculated in an operating circuit provided in the control device. Then, the speed of the vehicle body is estimated on the basis of the highest wheel speed among the wheel speeds of the all the wheels. Since it is possible to judge whether or not the wheel is in the tendency of being locked, or whether or not the same is in the tendency of being released from the locked state, by the comparison between the estimated speed of the vehicle body and the wheel speed, the hydraulic pressure to be fed to the wheels can be controlled based on the result of the comparison.
In the above-described conventional arrangement, however, there may be an inconvenience when a wheel having a diameter smaller than that of other wheels is used in emergency cases, such as for replacing a flat tire, etc. Specifically, the wheel of the smaller diameter rotates faster than the other wheels, and therefore the wheel speed of this wheel is determined to be the highest wheel speed, resulting in an excessive estimation of the speed of the vehicle body. Consequently, as a result of the comparison between the overestimated speed of the vehicle body and the wheel speed of the other wheel(s), the other wheel(s) are erroneously judged to be slipping, whereby unnecessary reduction of the hydraulic pressure takes place, or repressuring after the antilocking operation is delayed.
In order to solve the above-described inconveniences in the simplest manner, such arrangement may be proposed that the second highest wheel speed, in place of the highest wheel speed, is always used to obtain the estimated vehicle speed. However, when the same wheel is mounted in every wheel, the accuracy of the estimated speed of the vehicle body is undesirably deteriorated.